I’m having the same issue with cylindrical prints. At first I thought it might have been because of some extra extrusion at the z seam, so was starting to do some tests to improve that when I noticed something a bit odd.
Making a 20mm diameter test cylinder with just 2 walls has shown that just after the z seam it does indeed get thicker, but it is the same the opposite side of the cylinder as well. Not only that but the 2 walls actually separate slightly. As best as I can tell from measurements, the outer wall bulges out a little and the inner bulges in a little.
You can see the bulges and the gap in the walls at the top left and bottom right of this print. My wall thicknesses are normally close to perfect, and between these bulges they are near enough spot on 0.8mm. But just before the bulge they drop down to 0.6mm and at the gap they are 1.1mm.
My next guess was this could be down to the xy calibration, but I’m a little confused about that too (see here Confused about xy calibration)